Proton

Proton, nuclear particle having a positive charge identical in magnitude to the negative charge of an electron and, together with the neutron, a constituent of all atomic nuclei. The proton is also called a nucleon, as is the neutron. A single proton forms the nucleus of the hydrogen atom. The mass of a proton is 1.6726 × 10^-27 kg, or approximately 1,836 times that of an electron. Consequently, the mass of an atom is contained almost entirely in the nucleus. The proton has an intrinsic angular momentum, or spin, and thus a magnetic moment. In addition, the proton obeys the exclusion principle. The atomic number of an element denotes the number of protons in the nucleus and determines what element it is. In nuclear physics the proton is used as a projectile in large accelerators to bombard nuclei to produce fundamental particles (see Particle Accelerators). As the hydrogen ion, the proton plays an important role in chemistry (see Acids and Bases; Ionization).

The antiproton, the antiparticle of the proton, is also called a negative proton. It differs from the proton in having a negative charge and not being a constituent of atomic nuclei. The antiproton is stable in a vacuum and does not decay spontaneously. When an antiproton collides with a proton or a neutron, however, the two particles are transformed into mesons, which have an extremely short half-life (see Radioactivity). Although physicists first postulated the existence of this elementary particle in the 1930s, the antiproton was positively identified for the first time in 1955 at the University of California Radiation Laboratory.

Protons are essential parts of ordinary matter and are stable over periods of billions and even trillions of years. Particle physicists are nevertheless interested in learning whether protons eventually decay, on a timescale of 10³³ years or more. This interest derives from current attempts at grand unification theories that would combine all four fundamental interactions of matter in a single scheme (see Unified Field Theory). Many of these attempts entail the ultimate instability of the proton, so research groups at a number of accelerator facilities are conducting tests to detect such decays. No clear evidence has yet been found; possible indications thus far can be interpreted in other ways.